1. Field of the Invention
The present invention relates to an internal mixer for kneading plastic materials, particularly synthetic materials, caoutchouc or rubber mixtures. The mixer includes a housing with a mixing chamber, two rotors which are driveable in opposite directions and are arranged in the mixing chamber with parallel axes. The rotors have non-meshing rotor bodies, wherein each rotor bodies has four helically extending mixing wings which at one end thereof define a front passage to the corresponding front wall of the housing.
2. Description of the Related Art
In an internal mixer, as it is known, for example, from EP 0 264 224 B1, each rotor body has two long wings and two short wings, wherein the long mixing wings extend over at least half the axial length of the mixing chamber and end at the discharge end thereof at the corresponding end face of the rotor bodies, while the long mixing wings are defined at their discharge ends by passages which have the appropriate width and extend to the respective front wall of the housing. The angle of inclination of the mixing wings, i.e., the angle of the mixing wings with the tangent of the rotor bodies, is about 60° to 70°.
In this internal mixer which is known as a rubber kneader, the energy introduction into the material to be mixed, i.e., predominantly caoutchouc mixtures, is good.
When mixing caoutchouc, a distinction is made between. dispersive mixing and distributive mixing. Dispersive mixing is the comminution of mixture components, for example, soot agglomerates. Distributive mixing is the macroscopic homogenizing of the mixture components within the mixing chamber.
The basic principle of the internal mixer, for example, described in EP 0 264 224 B1, is the fact that the caoutchouc mixture to be mixed is intensively mixed within a bulge forming in front of the active side of the long mixing wings and, due to the angle of the helix, i.e., the angle of the mixing wings relative to the axis of the rotor, and due to the rotation of the rotor bodies, the material is transported axially until it reaches the passage at the free front end in axial direction.
As tests carried out by the applicant have shown, depending on the mixture only a small quantity of the caoutchouc mixture relative to the total mixture located in the mixing chamber travels through the gap between the mixing wing tips on the rotor bodies and the mixing chamber wall. It is possible to conclude from this that the dispersive mixing effect essentially results from the movement of the caoutchouc mixture in front of the active side of the long mixing wings.
As disclosed in EP 0 264 224. B1, after the caoutchouc mixture passes through the free passage at the end face, the mixture is deflected axially by the subsequent short wings, so that the caoutchouc mixture is supplied to the next following long mixing wing, where the mixing process is repeated in front of the active side. At this stage, the material once again predominantly flows axially along the long mixing wings and simultaneously is moved, rotationally within a bead. In the middle of the mixing chamber, where the rotor bodies move past each other in a tangential direction, the caoutchouc mixture is transferred or exchanged from one mixing chamber half into the other mixing chamber half. The flow process of the material in one mixing chamber half as well as the material exchange between the rotor bodies results in an effective distributive mixing effect.
Consequently, the dispersive mixing effect of a rotor body depends predominantly on the number of the long mixing wings which are available, the shape of the mixing wing geometry, the configuration of the three passages at the front end, the geometric arrangement of the mixing wings on the rotor bodies, and various method-related operating parameters of the internal mixer, such as, for example, filling factor, rate of rotation, etc. These factors continue to determine the quantity of the energy which can be introduced into the material being mixed with respect to a given mixing period. It is generally known that dispersive mixing effect and energy introduction correlate. With increasing energy introduction into a material being mixed the dispersive mixing effect and, thus, the quality of the manufactured caoutchouc mixture, are increased. The dispersive mixing effects of rotor bodies and, thus, the energy which can be introduced into the caoutchouc or rubber substance is not of particular importance for the manufacture of basic mixtures which do not yet contain or do not yet contain all cross-linking chemicals.
The distributive mixing effect of a rotor body also depends predominantly on the number of available long and short wings. With increasing number of mixing wings on a rotor body, the caoutchouc mixture is increasingly divided into smaller portions and is axially deflected. Moreover, inter alia, the arrangement of the mixing wings, the angle of the helix, the configuration of the free passages at the front end, possibly also at both ends and the selection of the process-related operating parameters are of great significance with respect to the distributive mixing effect.